Exploration drilling typically involves drilling to subterranean depths of thousands of meters. Accordingly, it is necessary to join and install successive sections of pipe or rod as the drill string is advanced into the well.
Drill rod, depending on their specific configuration, may weigh between ten to twenty kilograms each and measure approximately two to three meters in length. Conventionally, the drill rods are interconnected by male and female threaded connections provided at the respective rod ends. Additionally, it is typically unavoidable to have to exchange the drill bit or other tools at the lowermost end of the drill string at regular intervals during drilling. This exchange process involves retrieving the entire drill string from the borehole, exchanging the lowermost portion and then reinstalling the entire drill string after which drilling may continue. In practice, and depending upon rock conditions, it is not uncommon for ten to twenty retrieval operations to be undertaken per drill hole. Accordingly, a very large number of drill rods are required to be handled and in particular taken from a transport or carriage carrier to the drilling rig where they are ready for axial alignment and coupling to the drill string. Of course, the reverse operation is also required during string retrieval. Example rod handling systems are disclosed in U.S. Pat. No. 3,043,619; GB 2334270; WO 00/65193; and WO 2011/129760.
A rod handling system may typically comprise a robot arm having a dedicated gripper for gripping the drill rods. During a forward drilling operation, the robotic arm is arranged to pick up drill rods at a transport or intermediate carrier and to place the drill rod in the drill rig, whereupon the drill rod is connected to an already installed drill rod to extend the drill string. During a drill string retrieval operation, the robotic arm is arranged to pick up disconnected rods from the drill rig and to replace them onto the transport or intermediate carrier.
In order to provide a fully automatic system, that eliminates the need for regular manual intervention, it is desirable for the rod handling system to be able to connect and disconnect the drill rod to/from the installed drill rods. However, the threads used in many drilling applications, including wire-line and core drilling, typically have a very low thread height, and are slightly conical. If a pair of such threads is brought together axially at random, experience shows that there is about 60% chance of the threads not engaging each other, or engaging each other incorrectly. In either case, the threads may become damaged, resulting in additional cost and work.
WO 02/079603A1 discloses a system for automatically connecting drill rods to and form a drill string. In this system, marks are provided around the perimeter of the rods, such that their rotational positions can be determined, thus allowing the rods to be rotationally aligned for optimal thread entry. However, existing systems of this type cannot guarantee alignment and there remains a risk of the rods and their threads being damaged by misalignment. Accordingly, there is a need for a rod handling system for interconnecting drill rods that addresses the above problems.